Polluting agents in exhaust gases generated by internal combustion engines and in stack gases generated by power plants burning fossil fuels have seriously degraded air quality in many locations and created acid rain, smog and a host of other environmental problems. Two pollution components commonly found in exhaust and stack gases are nitrogen compounds such as nitric oxide and nitrogen dioxide (collectively called NOx), carbon monoxide and various hydrocarbon compounds which result from incomplete or imperfect combustion of the fossil fuels. Although these components are known to cause pollution problems, recent attention has focused on NOx compounds which are in part responsible for acid rain. Attempts to control, reduce or eliminate the pollutants in exhaust gases have included mechanical, electrical and chemical apparatus.
One known method of reducing pollutants is by means of chemical apparatus such as catalytic converters. Such devices are typically used with internal combustion engines such as those found in automobiles. In a catalytic converter system, engine exhaust gases are mixed with air and the mixture then passes through a catalyst bed. In the catalyst bed, pollutants in the exhaust gas react with the air in the mixture to produce more complete combustion. Such devices have drawbacks. For example, they restrict the exhaust gas flow and therefore increase the exhaust back-pressure on the internal combustion engine, resulting in reduced efficiency and fuel economy. Further, the catalyst must be replaced at regular intervals to maintain conversion efficiency, thus increasing maintenance expense. In addition, such catalytic converters are not known to efficiently reduce NOx pollutants. Consequently, additional techniques are generally used in conjunction with the converter systems to reduce NOx pollutants. These additional techniques generally take the form of recirculating a portion of the engine exhaust gases through the engine to reduce combustion temperatures. However, these additional techniques further reduce fuel efficiency and economy and require additional equipment.
Other proposals to reduce pollution to a satisfactory level have involved the application of electric fields to the pollutant-containing exhaust gas. Such proposals involve mixing air with the exhaust gases, and then subjecting the mixture to an electric field which may take the form of a spark, an electric arc, electric plasma, a corona or a silent discharge. The electric field must be sufficiently intense and of sufficient duration to create active species such as free radicals, ionic species, ion-radicals, metastable neutral species and very reactive molecules such as ozone within the exhaust gas which then react with the pollutants to dissociate the compounds or to form harmless compounds. Two attempts to use electrical discharge to reduce pollutants are shown in U.S. Pat. Nos. 3,188,167 and 3,979,193.
Previously known electrical devices appear to have additional drawbacks. First, because of the intense electric field used in such devices, oxidation of nitrogen may actually increase the amount of NOx pollutants. Second, due to their construction, such devices typically increase exhaust back pressure, thereby reducing fuel economy and efficiency. Moreover, the addition of secondary air requires an air compressor, and this is a disadvantage because of the additional energy consumed.
It is an object of the present invention to provide a simple, efficient and economical exhaust gas treatment system in which exhaust gases are treated with a specially selected electric discharge to reduce harmful pollutants such as carbon monoxide, hydrocarbon and NOx emissions without producing significant back pressure in the exhaust system.